A cellulose acetate, particularly cellulose triacetate has been used to prepare various plastic products, such as a film or a fiber because the cellulose acetate is excellent in dimensional stability and heat-resistance. A cellulose acetate film is a representative photographic support. Further, the cellulose acetate film has an optical isotropy. Accordingly, the film is also used in a liquid crystal display device, which has recently extended its market. The cellulose acetate film is used as a protective film of a polarizing plate or a color filter in the liquid crystal display device.
A cellulose acetate product is prepared by using a solution (that is called "dope") or melt of cellulose acetate. For example, a cellulose acetate film is prepared according to a solvent cast method or a melt cast method. The solvent cast method comprises the steps of casting a solution of cellulose acetate in a solvent on a support, and evaporating the solvent to form a film. The melt cast method comprises the steps of casting molten cellulose acetate on a support under heating, and cooling it to form a film. The solvent cast method can form a highly flat film, compared with the melt cast method. Therefore, the solvent cast method is generally employed to give a cellulose acetate film.
The solvent used in the solvent cast method must have functions not only of dissolving the cellulose acetate but also of forming an excellent film. In more detail, the viscosity and the polymer concentration of the solution (dope) should be appropriately adjusted to form a flat plane film having a uniform thickness. The dope also should have enough stability. Further, the dope should easily be set to gel. Furthermore, the formed film should easily be peeled off the support. The most appropriate solvent must be selected to satisfy these requirements. Moreover, the solvent should be so easily evaporated that the solvent scarcely can remain in the film.
Various organic solvents have been proposed as the solvents of cellulose acetate. However, only methylene chloride satisfies all the above-mentioned requirements. Accordingly, solvents other than methylene chloride have not been practically used.
However, the use of hydrocarbon halides such as methylene chloride has recently been restricted severely to protect the global environmental conditions. Further, methylene chloride is apt to vaporize in the process for the preparation of the film, because it has a low boiling point (41.degree. C.). Accordingly, methylene chloride may cause problems in the working environment. Therefore, the process is conducted under closed conditions. However, there is a technical limitation on sealing methylene chloride in a closed system.
By the way, acetone and methyl acetate are widely used organic solvents. Acetone and methyl acetate have an appropriate boiling point (56.degree. C. and 57.degree. C. respectively). The process of evaporating acetone or methyl acetate does not need a large thermal energy. Further, acetone and methyl acetate have few problems on the human body and the global environmental conditions, compared with the organic chloride solvents.
However, cellulose acetate has a poor solubility in acetone or methyl acetate. Cellulose triacetate having a degree of substitution of not more than 2.80 (acetic acid content: 60.1%) is slightly soluble in, and merely swelled in and acetone or methyl acetate.
C. J. Malm et al. report in Ind. Enig. Chem., 43 (1951) 688, that solvents for cellulose acetate are limited in number, compared with cellulose propionate or cellulose butyrate. Cellulose propionate or cellulose butyrate is soluble in a ketone or an ester, in which cellulose acetate is insoluble. However, cellulose propionate and cellulose butyrate films are inferior to a cellulose acetate film in the mechanical strength and the durability.
Cellulose acetate propionate and cellulose acetate butyrate are commercially available. For example, a catalogue of Eastman (June 1994) shows various cellulose acetate propionates and cellulose acetate butyrates, most of which are soluble in widely used organic solvents such as acetone and methyl acetate. However, cellulose acetate propionate and cellulose acetate butyrate films are still inferior to a cellulose acetate film in the mechanical strength and the durability. The cellulose acetate propionate shown in the catalogue is not used in a protective film or a photographic support (which requires a high mechanical strength), but is used in printing inks.
Japanese Patent Provisional Publication No. 6(1994)-501040 proposes a melt casting method in place of the solvent casting method having the problems mentioned above. However, the melt casting method has another problem that the melting point of cellulose triacetate is higher than the decomposition point. If cellulose triacetate, which has a high degree of acetyl substitution, is heated, it would be decomposed before melted. The invention in the publication adjusts the degree of acetyl substitution in the range of 1.9 to 2.6 to solve the problem of decomposition. The publication further discloses cellulose acetate propionate having a degree of propionyl substitution in the range of 0 to 0.9. In the Example B of the publication, cellulose acetate propionate has the degree of acetyl substitution of 1.90 and the degree of propionyl substitution of 0.71. In the Example C of the publication, cellulose acetate propionate has the degree of acetyl substitution of 2.10 and the degree of propionyl substitution of 0.50.